Domain-Specific Languages Guide

During the last years of the 00's I got deeply interested in the resurgence of DSLs, fueled by efforts in the Ruby community, the desire to create fluent interfaces, and the enticing, if hazy, prospect of Language Workbenches. I saw a fair few projects using ideas from DSLs, but most people didn't have a good sense of their options in building and using them well. So I decided to tackle this by writing a book of patterns to use for simple internal and external textual DSLs. While much of that interest has abated during the intervening years, I still feel this book does a good job of capturing these techniques. In particular, it shines a light on using adaptive models to capture alternative computational models, such as rules engines, decision tables, and state machines.

I was asked to put together a discussion of DSLs for non-technical types. Maybe I've been reading too much Stephen O'Grady, but I felt an irresistible urge to do it in a Q and A manner. So here it comes.

When the topic of DomainSpecificLanguage comes up, one of the common puzzles is exactly what is or isn't a DSL. The trouble is that there is no precise definition for a DSL and there is a large gray area between DSLs and other things.

Most new ideas in software developments are really new variations on old ideas. This article describes one of these, the growing idea of a class of tools that I call Language Workbenches - examples of which include Intentional Software, JetBrains's Meta Programming System, and Microsoft's Software Factories. These tools take an old style of development - which I call language oriented programming and use IDE tooling in a bid to make language oriented programming a viable approach. Although I'm not enough of a prognosticator to say whether they will succeed in their ambition, I do think that these tools are some of the most interesting things on the horizon of software development. Interesting enough to write this essay to try to explain, at least in outline, how they work and the main issues around their future usefulness.

Most of our software logic is written in our programming languages, these give us the best environment to write and evolve such logic. But there are circumstances when it's useful to move that logic into a data structure that our imperative code can interpret - what I refer to as an adaptive model. Here I'll show some product selection logic in JavaScript and show how it can be refactored to a simple production rule system encoded in JSON. This JSON data allows us to share this selection logic between devices using different programming languages and to update this logic without updating the code on these devices.

Will DSLs allow business people to write software rules without involving programmers?

When people talk about DSLs it's common to raise the question of business people writing code for themselves. I like to apply the COBOL inference to this line of thought. That is that one of the original aims of COBOL was to allow people to write software without programmers, and we know how that worked out. So when any scheme is hatched to write code without programmers, I have to ask what's special this time that would make it succeed where COBOL (and so many other things) have failed.

A common phrase that's bandied about when talking about DomainSpecificLanguages (or indeed any computer language) is that of noisy syntax. People may say that Ruby is less noisy than Java, or that external DSLs are less noisy than internal DSLs. By Syntactic Noise, what people mean is extraneous characters that aren't part of what we really need to say, but are there to satisfy the language definition. Noise characters are bad because they obscure the meaning of our program, forcing us to puzzle out what it's doing.

My book on Domain Specific Languages uses a pattern structure to organize the concepts that I use to describe DSLs. This catalog is a list of the patterns in that book. Each is linked to a page for each pattern. While the pages don’t contain very much detail, they do provide a reference URI for each pattern.